license field in setup.cfg should not accept file:

[pganssle]

As brought to my attention by this comment, it seems that in setup.cfg, license accepts a file:, and the docstring of _parse_file even mentions file: LICENSE. I think this was based on a misunderstanding of what the license file does, and I think will lead to problems with #1390 if actually used this way.

We should remove support for file: in the license field.

To do on this:

• Remove file: support from the code. I'd start with just removing it and seeing what error is raised if you have file: in there, but if the error message is confusing, we need to add a "throw error if it's file:" parser.
• Remove file: from the setup.cfg documentation.
• Make sure there's a test using something like license: Apache 2.0
• Add a test that license: file: throws an exception (Maybe?)

[RajdeepRao]

I'd like to work on this as a part of the bloomberg pypa sprint